Paint compositions especially adapted for the coating of automotive glass require a number of specific properties for commercial acceptance. The paint compositions must possess the required color and opacity. Furthermore, the coating must be scratch resistant. It is also of benefit if the paint composition does not stick to the molds which can be used to form the windshield glass. The paint composition should also be readily usable in a variety of printing mediums including UV-curable, water-miscible, thermoplastic and conventional screen printing mediums.
It is known in the art to employ a flux composition containing major amounts of zinc oxide, boron oxide, silica and alkali oxides for the decorative coating of light bulbs. Such compositions are required to withstand resistance to the heat produced from the light source. There is, however, no requirement that such coatings be scratch resistant.
Traditional windshield coatings utilize lead oxide as a major flux. The presence of lead oxide, from an engineering standpoint, is advantageous. It provides for a smooth surface, excellent gloss, and low melting temperatures. Lead oxide is a powerful flux and therefore minimizes the need for alternative fluxes such as alkali oxides. This is an advantage because alkali oxides as known by those skilled in the art, always raise the coefficient of thermal expansion (C.T.E.) of the glass. If the C.T.E. of the coating surpasses that of the substrate, tensile stresses are built up in the coating during the cooling stage of the firing process. Since glass is very weak in tension, this could lead to fracture or "crazing" of the windshield.
Lead oxide containing glasses tend to have low viscosities and low surface tensions. Lead oxide containing glasses usually have wide firing ranges, thus reducing the propensity for over firing or under firing. These properties of the traditional lead oxide containing systems facilitate the healing of any defects, thus providing a superior surface finish.
Despite the advantages obtained utilizing a flux containing lead oxide, there are well documented health and environmental hazards. For these reasons, it was deemed necessary to develop a lead-free flux system that could provide the essential properties originally obtained through the use of lead. These properties are:
1) Completely fire at low temperatures (.about.1060.degree. F. for 7 minutes).
2) Have a firing "window" of at least 100.degree. F.
3) Have a Coefficient of Thermal Expansion (C.T.E.) of lower than 92.times.10.sup.-7.
4) Have adequate chemical durability to withstand industry wide tests that simulate reasonable exposure to the environment.
5) Have very good scratch resistance in order to withstand the normal handling encountered during the storage and installation of the windshields within the cars.
6) Allow the option of overprinting with conductive silver pastes for antennas or defroster grids.
It would be a significant advance in the art of providing automobile windshields with protective coatings if a lead-free coating could be made having the above properties.